Yesterday, I downloaded the LM21 (Cinnamon) ISO.

This morning, I burned the ISO to a thumb drive and installed LM21 on my laptop - successfully!

I applied my documented "tweaks" and everything still works!

I backed up my new install, via Rescuezilla 2.3.1 (Focal), without incident.

Stay tuned...

I shouldn't; but I will ask. Is this your first time?!

2nd. If at first you don't succeed...

Writing this to cheer you up. Maybe this was meant for the other thread.

Coming back to this, I spoke too soon. Though I am experienced in installing, I am not experienced in upgrading. This may have been my first time too, apart from upgrading a Debian distro or two to unstable/experimental as far as I remember.

Last night I tried to upgrade from Mint 20.3 to Mint 21. After a wild search I managed to find the 21 repos and added them to the sources. I also left the 20.3 sources intact which is not recommended.

I had a problem with the casper package, which I had to force install with dpkg. Then I regretibly rebooted while I still had some 200 plus packages to be upgraded. I had a kernel panic, but at least the last kernel booted the system.
I have four kernels listed in grub. 5.4.0-126-generic, 5.4.0-122-generic, 5.4.0-109-generic and 5.4.0-74-generic. The first one (126) ends up in kernel panic. The next two (122 and 109) show a message "kernel was not found; you need to load the kernel first". The last one is able to boot, for the moment.

Today I continued the upgrade only to find conficting packages (unmet dependencies and this breaks that). After some manual installing I have only one package that is left out (network-manager-gnome). Still, after a reboot, the kernel situation remained the same. Then after another reboot I ended up with kernel panic on the last kernel (74).

I can boot the system through grub and going to a terminal. And WiFi is working. I spend a few hours trying to fix things but I got stuck at creating the System.map-kernel and config-kernel files which I removed (from one test kernel).

I will end up probably installing the newer version from USB as you did. Since the computer has two identical systems I can have another try for fun. But later.

Update. When I boot to the terminal I use another kernel that is in /boot, 5.15.0-52-generic. I tried "startx" and was able to boot into the GUI. Maybe there is hope after all.
Then I used the Update Manager application and installed kernels 5.13*, 5.11* and 5.8*, since I am missing some commands to do that from the CLI. The kernels boot to the terminal, no GUI, but I can start it with startx. Some of the searches I have done were indicating to lightdm which is a broken package now but I will try to fix that too. I must have more broken packages, maybe I should remove Mint 20.3 repos or go for a new install.

It took me at least 2 hours last night due to slow download, and at least 3-4 hours today for all this. A new installation would have been much quicker.

Love it when a plan comes together. Congratulations.

Trial amd error are my constant companions also.

I appreciate your zeal, AND your patience, Debian6to11, but there's got to be a better way! What computer are you running, and what type of internet connection are you using?

I'm running an older Samsung 500GB laptop on a Spectrum cable network and it takes me 8-10 minutes to backup or restore my entire system!

My laptop is a very essential tool, used for data mining and recordkeeping, so it's vital that it always be operational.

This one is a fourth gen i5, an HP Elitebook 850 G2. It is nice but my older HP 650 second gen dual core Pentium is faster with Debian believe it or not.

I am far away from the router and my WiFi is from a USB. It keeps disconnecting and reconnecting and sometimes it goes under 100KB/s while the max is just over 1MB/s. It does not bother me as I am watching YouTube videos when I am working on my laptops.

And I have an update. I upgraded the other Mint 20.3 to 21 without many problems. Some manual installation was required and it did took me some time, around 3 hours with checking everything. And I found out the lighdm package problem, it was caused when package packagekit was upgraded on the first system. You can see below why it was removed, I did not pay enough attention the first time. It is the outcome of the second system. Six packages did not upgrade and Mint 21 is running very smooth.
As far as the first upgrade it was a simple fix. I installed a different window manager, gdm3, and everything is good there too apart from missing Firefox. It is just the backup system which I do not use much. And there are other browsers in case of emergency.
Edit. I made this one identical to the "good upgrade". The solution was to downgrade packagekit and then I was able to install the removed packages listed above, including firefox and lightdm. /Edit

The only way to learn.

I admire your tenacity, Debian6to11. Good luck!